[fix] nmea callback: remove \0 bytes to avoid CDR crash

[remco-r]

In my setup (moving base 2x ublox F9P, ROS2 Jazzy) RTCM messages are published on the same UART channel as connection to my PC. This driver currently wrongly interprets this as NMEA.
'\0' bytes in the RTCM messages will come in. and are processed by FastCDR to the ROS middleware. The new FastCDR 2.2.7, throws when serializing '\0' bytes to characters, making the driver crash.

terminate called after throwing an instance of 'eprosima::fastcdr::exception::BadParamException'
  what():  The string contains null characters

This fixes it, but proper handling of RTCM messages is adviced.
Valid NMEA is ASCII and never contains \0 byte, legitimate NMEA will not be deleted.

publish.nmea: false should also avoid the crash.